Multistage-compression heat pump cycles have been hitherto known which increases the pressure of refrigerant through multiple stages so as to improve a cycle efficiency (COP) of a heat pump cycle (vapor compression refrigerant cycle).
For example, Patent Document 1 (U.S. Pat. No. 5,704,219 corresponding to JP 9-86149A) discloses the so-called gas injection cycle (economizer refrigerant cycle) which includes a compressor with a suction port for sucking refrigerant, a discharge port for discharging refrigerant, and an intermediate pressure port for allowing an intermediate-pressure refrigerant in the cycle to flow thereinto and for mixing the refrigerant with another refrigerant whose pressure is being increased.
Such a gas injection cycle compressor increases the pressure of the refrigerant through two compression stages, specifically, a low-pressure side compression stage for increasing the pressure of the low-pressure refrigerant into an intermediate-pressure refrigerant, and a high-pressure side compression stage for increasing the pressure of the intermediate-pressure refrigerant into a high-pressure refrigerant. Thus, the pressure of the intermediate-pressure refrigerant flowing from the intermediate-pressure port is adjusted to an appropriate level, so that the compression efficiency of the compressor can be improved to thereby improve the cycle efficiency.
The gas injection cycle disclosed in Patent Document 1 is applied to an air conditioner for a vehicle, and thus designed to heat feed air as a fluid for heat exchange by exchanging heat between a high-temperature and high-pressure refrigerant discharged from the compressor and the feed air blown into a vehicle interior by a user-side heat exchanger (indoor condenser) upon heating the vehicle interior as a space of interest for air conditioning.
In a normal operation, the user-side heat exchanger can exhibit enough heating capacity to increase the temperature of the feed air to a target temperature required for heating the vehicle interior. At this time, the opening degree of a high-pressure side expansion valve for decreasing the pressure of the refrigerant flowing from the user-side heat exchanger is adjusted to achieve the maximum cycle efficiency. In contrast, the user-side heat exchanger sometimes lacks the heating capacity for increasing the temperature of the feed air to the target temperature. In this case, the opening degree of the high-pressure side expansion valve is increased as compared to the normal operation.
Thus, it is required that in the normal operation, the cycle is operated while achieving the high cycle efficiency, while upon lacking the heating capacity, the heating capacity of the user-side heat exchanger is to be improved by increasing the flow rate of refrigerant (amount of gas injection) flowing thereinto from the intermediate-pressure port of the compressor.